Compact multipass optical isolator

ABSTRACT

An optical isolator for transmitting light in a first direction along an optical pathway and blocking light in a second direction along an optical pathway. The optical isolator includes an input polarizer having a pass axis at a first angle, an output polarizer having a pass axis at a second angle, a Faraday rotator material between the polarizers having a Verdet constant and an axis of maximum length therethrough, generation means for generating a magnetic field around and inside the rotator material, and at least one reflector configured to define an optical length through the rotator material which is longer than the axis therethrough. The optical pathway length through the rotator material, the magnetic field strength, and the Verdet constant are selected so as to rotate light through the Faraday rotator material from the first angle to the second angle.

REFERENCE TO PENDING PRIOR PATENT APPLICATION

This patent application claims benefit of pending prior U.S. ProvisionalPatent Application Ser. No. 60/454,223, filed Mar. 13, 2003 by KevinKnopp et al. for COMPACT MULTIPASS OPTICAL ISOLATOR (Attorney's DocketNo. AHURA-8 PROV), which patent application is hereby incorporatedherein by reference.

FIELD OF THE INVENTION

This invention is related to free space optics and fiber opticapplications apparatus and methods in general, and more particularly toapparatus and methods for optical isolation of light.

BACKGROUND OF THE INVENTION

Optical isolators are key elements required in most free space optic andfiber optic applications. The most commonly used type of isolator isbased around a Faraday rotator crystal. FIG. 1 is a schematic diagramthat illustrates a typical configuration of a conventional opticalisolator 5. An input polarizer 10 of isolator 5 is typically aligned toa linear polarization angle 15 of input light 20. A crystal 25 of agiven length L₁ which is immersed in a magnetic field generated by apair of magnets 30, which rotates the polarization state of the launchedlight 20 via the Faraday effect to an angle of 45⁰. An exit polarizer 35is then aligned to a non-parallel polarization angle 40 so as totransmit this polarization state at the angle of 45⁰ and pass light 20.Light 45, which has exited isolator 5 that may be reflected back intoisolator 5, passes through the exit polarizer 35 experiences anadditional rotation of polarization of 45⁰. The polarization state oflight 45 is then orthogonal to input polarizer angle 15 at the entranceplane to isolator 10. Optical isolation is thereby achieved in anoptical system before and after isolator 5.

In many applications, the size of an optical isolator assembly isextremely important. The size has traditionally been limited by theVerdet constant of the isolator's Faraday material. In shorterwavelength applications of less than 1000 nm, the Verdet constant is lowfor optically desirable (e.g., low optical loss) materials such asTerbium Gallium Granite (TGG). Hence, a long optical path length isneeded. Isolators with physical lengths of greater than 2 cm are commonfor these shorter wavelength applications. Accordingly, it would be mostdesirable to obtain an optical isolator for shorter wavelengthapplications with a more compact physical size.

Referring to FIG. 2, there is shown an optical element 5A which canrotate plane of polarization of polarized light 20A in a non-reciprocalmanner, using Faraday-effect, which is called a Faraday rotator and isan essential building block for various optical devices such asisolator, circulator, and optical switch. A Faraday rotator 5A includesan optical element 25A and a magnet 30A surrounding the optical element20A.

The magnet is magnetized and positioned in such a way that its magneticfield is aligned with an optical axis of the optical element. As aresult, plane of polarization of polarized light from a source such as alaser or an optical fiber traveling along the optical axis of theelement will be rotated by a desired angle θ. This rotation may beclockwise or counterclockwise and its magnitude depends on Verdetconstant V of optical element, magnetic field strength (B), and lengthof optical element (L). This rotation is expressed as:θ=VBL

During past several years various materials have been identified andmade which can be used as an optical element for Faraday rotation. Amongthem Bismuth-Iron-Garnet composition (BIG) and TGG crystal are the mostwidely used materials. The value of Verdet constant, normally depends onthe wavelength of the incident light and temperature, thus for a givenwavelength and length of crystal a specific magnetic field B is neededto achieve a desired rotation angle θ. However, for BIG material theaforementioned linear equation is valid only up to certain level ofmagnetic flux strength, e.g., where B<350 Oe, and eventually therotation angle will saturate and become constant by increasing themagnetic flux density B. This is an interesting feature for devicesbased on BIG material. As long as the magnetic field remains above theminimum saturation field the rotation angle will not change with anydisturbance to B field due to temperature or proximity to other magneticmaterials. Commercial single stage optical isolators with better than 40dB isolation, less than 0.5 dB transmission loss, and few nano-meterband-width at 1550 nm are readily available. A major draw back for BIGmaterials is that their window of optical transparency is limited toabove 1100 nm wavelength range and in visible and Near-IR wavelength(less than 1000 nm) BIG has large optical absorption loss and is notusable.

Magneto-optic crystals such as TGG crystal show very small opticalabsorption over large wavelength range including visible and NIR.However, magneto-optic crystals suffer from three fundamental problems.First, magneto-optic crystals have low Verdet constant compared to BIG.For example, the Verdet constant for TGG is two orders of magnitude lessthan that of BIG. Second, the rotation angle of magneto-optic crystalsremains linearly proportional to B field for practical values of Bfield. This implies that a long crystal, e.g., on the order of tens ofcentimeters, along with a strong magnetic field (close to 1 Tesla) isneeded in order to get about a 45° polarization rotation angle. Inaddition, to maintain constant rotation angle over life of the device,one has to make sure that the B field will not change by aging ordisturbed by external perturbations such as temperature or proximity toother magnetic materials. Third, the use of multistage isolators arerequired to achieve isolation better than 30 dB. Such multistageisolators add to the cost and make the size of the isolator almostimpractical for most applications.

SUMMARY OF THE INVENTION

To overcome size issue the present invention uses a volume of crystalseveral times by reflecting the incoming light inside the crystalseveral times and, as a result, increases the effective interactionlength by the number of bounces within crystal. For example, for a givenmagnetic field strength, if light bounces m times inside a TGG crystalof length L, the effective interaction length will be mL. Thus, requiredlength will be m times shorter in order to achieve the same rotationangle.

An object of the invention is to provide a compact, low cost, andmanufacturable optical isolator with a better than 40 dB isolation usingmagneto-optic crystals such as TGG.

Another object of the invention is to provide an optical isolator whichcan be implemented at any desired wavelength, particularly at 976 nm,980 nm and 880 nm.

A further object of the invention is to provide an optical isolatorwhich overcomes the large size, cost and sensitivity to magnetic fielddistortion which have been major bottlenecks for commercialization ofisolators based on TGG crystals.

With the above and other objects in view, as will hereinafter appear,there is provided an optical isolator for transmitting light in a firstdirection along an optical pathway therethrough and blocking the lightin a second direction along the optical pathway, and the first directionand the second direction being in opposition to one another, the opticalisolator comprising:

an input polarizer and an output polarizer, the input polarizer having afirst pass axis of a first given angle, the output polarizer having asecond pass axis of a second given angle, the input polarizer configuredto polarize the light entering into the optical pathway to a first givenplane of polarization parallel to the first given angle;

a Faraday rotator material disposed between the input polarizer and theoutput polarizer, the Faraday rotator material having a given Verdetconstant, a first end and a second end in opposition to one another, thefirst end and the second end disposed at a maximum linear distanceacross the Faraday rotator material from one another, and the first endand the second end defining an axis therebetween defining a maximumlinear length through the Faraday rotator material;

generation means for generating a magnetic field around and inside theFaraday rotator material, the generation means providing a givenmagnetic field strength; and

at least one reflector configured along the optical pathway from theinput polarizer to the output polarizer, the at least one reflectordefining a given optical length of the optical pathway through theFaraday rotator material, and the given optical length through theFaraday rotator material being longer than the maximum linear distanceacross the Faraday rotator material;

wherein the given length of the optical pathway through the Faradayrotator material provided by the at least one reflector, the givenmagnetic field strength provided by the generation means, and the Verdetconstant of the Faraday rotator material are selected with respect toone another so as to rotate the light along the given length of theoptical pathway through the Faraday rotator material from the firstgiven angle of the input polarizer to the second given angle of theoutput polarizer.

In accordance with a further feature of the invention there is provideda method of optically isolating light by allowing transmission of thelight in a first direction along an optical pathway through an opticalisolator and blocking transmission of the light in a second directionalong a second direction through the optical isolator, and the firstdirection and the second direction being in opposition to one another,the method comprising:

initially polarizing the light with an input polarizer, the light beingpolarized at a first given plane of polarization parallel to a firstgiven angle;

transmitting the initially polarized light along an optical pathwaythrough a Faraday rotator material having a magnetic field appliedthereto so as to rotate the initially polarized light from the firstgiven angle to an intermediate angle;

reflecting the polarized light to provide a given number of passesthrough a portion of the Faraday rotator material so as to furtherrotate the polarized light from the intermediate angle to a second givenplane of polarization parallel to a second given angle; and

passing the polarized light at the second given plane of polarizationparallel to the second given angle through an output polarizer;

wherein the polarized light is reflected along the optical pathwaybetween the input polarizer and the output polarizer so as to provide anappropriate length of the optical pathway with a reduced length of theFaraday rotator material.

In accordance with a further feature of the invention there is provideda method of optically isolating light, the method comprising:

providing an optical isolator for transmitting light in a firstdirection along an optical pathway therethrough and blocking the lightin a second direction along the optical pathway, and the first directionand the second direction being in opposition to one another, the opticalisolator comprising:

-   -   an input polarizer and an output polarizer, the input polarizer        having a first pass axis of a first given angle, the output        polarizer having a second pass axis of a second given angle, the        input polarizer configured to polarize the light entering into        the optical pathway to a first given plane of polarization        parallel to the first given angle;    -   a Faraday rotator material disposed between the input polarizer        and the output polarizer, the Faraday rotator material having a        given Verdet constant, a first end and a second end in        opposition to one another, the first end and the second end        disposed at a maximum linear distance across the Faraday rotator        material from one another, and the first end and the second end        defining an axis therebetween defining a maximum linear length        through the Faraday rotator material;    -   generation means for generating a magnetic field around and        inside the Faraday rotator material, the generation means        providing a given magnetic field strength; and    -   at least one reflector configured along the optical pathway from        the input polarizer to the output polarizer, the at least one        reflector defining a given optical length of the optical pathway        through the Faraday rotator material, and the given optical        length through the Faraday rotator material being longer than        the maximum linear distance across the Faraday rotator material;    -   wherein the given length of the optical pathway through the        Faraday rotator material provided by the at least one reflector,        the given magnetic field strength provided by the generation        means, and the Verdet constant of the Faraday rotator material        are selected with respect to one another so as to rotate the        light along the given length of the optical pathway through the        Faraday rotator material from the first given angle of the input        polarizer to the second given angle of the output polarizer;

polarizing the light entering the input polarizer to the first givenangle;

rotating the polarized light from the first given angle to the secondgiven angle through the Faraday rotator material; and

passing the polarized light from the Faraday rotator material throughthe output polarizer so as to prevent reflected light from transmissionthrough the input polarizer due to a non-reciprocal rotation of thelight in the second direction through the Faraday rotator material so asto allow the input polarizer to block the reflected light.

The above and other features of the invention, including various noveldetails of construction and combinations of parts and method steps willnow be more particularly described with reference to the accompanyingdrawings and pointed out in the claims. It will be understood that theparticular devices and method steps embodying the invention are shown byway of illustration only and not as limitations of the invention. Theprinciples and features of this invention may be employed in various andnumerous embodiments without departing from the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention will bemore fully disclosed or rendered obvious by the following detaileddescription of the preferred embodiments of the invention, which are tobe considered together with the accompanying drawings wherein likenumbers refer to like parts, and further wherein:

FIG. 1 is a schematic diagram of one configuration of a conventionaloptical isolator;

FIG. 2 is a Faraday rotator with an optical element and a magnetsurrounding the optical element;

FIG. 3 is a schematic diagram illustrating a compact multipass opticalisolator of a preferred embodiment of the present invention;

FIG. 4 is a schematic diagram illustrating an integrated multipass andmultistage optical isolator of a preferred embodiment of the presentinvention;

FIGS. 5A and 5B are schematic diagrams illustrating a multipass opticalisolator of a preferred embodiment of the present invention;

FIGS. 6A and 6B are schematic diagrams illustrating a multipass opticalisolator having a single stage and a multipass isolator having asemi-double stage, respectively; and

FIGS. 7A and 7B are schematic diagrams illustrating a multipass isolatorhaving a cylindrical Faraday rotator material and a magnet surroundingthe cylindrical Faraday rotator material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 3, and in a preferred embodiment of the presentinvention, there is shown a novel multipass optical isolator 50, whichuses multiple optical passes 55 to reduce the physical size whilemaintaining the needed optical path length from input polarizer 60 tooutput polarizer 65.

Faraday rotator material 70 of optical isolator 50 has a pair ofopposing facets 75 which are covered with a high reflecting coating 78so as to form a multipass etalon 80. A small uncoated region 85 ispatterned on each facet 75 to allow light 90 to enter and exit etalon80.

Referring still to FIG. 3, and in a preferred embodiment of the presentinvention, the angle of incidence α of light 90 to Faraday rotatoretalon 80 is selected to set the desired number of reflections off ofhigh reflecting coating 75 which in turn allows the desired number ofpasses through Faraday rotator material 70. One or more magnets 95 areconfigured adjacent to Faraday rotator material 70 so as to generate amagnetic field. Length L₂ of Faraday rotator material 70 is selected inaccordance with the given incident angle α, the strength of the magneticfield generated by magnets 95, the Verdet constant of Faraday rotatormaterial 70, and the number of bounces 55, such that length L₂ causesthe total rotation of the polarization state is 45° from input polarizer60 to output polarizer 65. This multipass configuration of opticalisolator 50 is possible because the direction of polarization staterotation obtained via the Faraday effect is set by the direction of themagnetic field as opposed to the direction of light propagation. Thus,multiple optical passes 55 through Faraday rotator material 70 amplifythe rotation effect which in turn allows a reduced physical length L₂ ofFaraday rotator material 70. Input polarizer 60 and output polarizer 65may be directly attached to rotator material 70 as the angle dependenceof the transmittance for a particular pass axis is low. Magnets 95 arepreferably either permanent poled magnetics or electromagnetic. Thephysical orientation of the magnetic field applied by magnets 95 isgoverned by the physics of the Faraday effect.

Referring now to FIG. 4, and in a preferred embodiment of the presentinvention, there is shown an integrated multipass and multistage opticalisolator 100 having a particular geometry for higher levels of opticalisolation. The geometry of multistage optical isolator 100 is obtainedusing three or more polarizers 105. A second midstage polarizer 105Aserves as the output polarizer of first stage of isolation 110 and theinput polarizer to the second stage 115 of isolation. The back surfaceof the polarizer is coated with a high reflection coating to form amirror 120 so as to redirect light 125 into the second stage 115.Compact isolator 100 preferably has an isolation of greater than 50 dB.The angle of incidence β of the light 125 to Faraday rotator material130 is selected to set the desired number of reflections which in turnallows the desired number of passes through rotator 130. Length L₃ ofFaraday rotator material 130 is selected in accordance with the givenincident angle β, magnetic field, Verdet constant, and number of bouncessuch that the total rotation of the polarization state is 45° from theinput polarizer 105A of first stage 110 to output polarizer 105B.Similarly, another rotation of 45° occurs second stage 115 to furtherrotate the polarization an additional 45°. Output light 135 of isolatedlight 125 from two stage unit is orthogonal in polarization to inputlight 140.

In addition to the two stage configuration as shown in FIG. 4, apreferred embodiment of the present invention comprises a multipass andmultistage isolator with at least three stages assembled together in asimilar manner.

Referring now to FIG. 5, and in a preferred embodiment of the presentinvention, there is shown a sandwich isolator 200 having a slab of TGGcrystal 205 sandwiched between two magnet slabs 210 with propermagnetization orientation. Incident light 215 is reflected by mirrors218 so as to travel along length L of crystal 205 several times whicheffectively increases the interaction length and minimizes requiredvolume and size of TGG crystal 205 for a desired rotation angle such as45°.

Referring now to FIGS. 6A and 6B, to build isolator 200 or a circulatorbased on a Faraday rotor material, it is required to add polarizers 220at proper locations. An important parameter for an optical isolator iscalled extinction ratio which is a measure of achievable isolation byisolator. A single stage isolator (FIG. 6A) and a semi-double isolator(FIG. 6B) are shown based on multi-pass Faraday rotator 200.

This structure can be extended to multistage isolators by adding towidth of the crystal and repeating the single stage configuration asmany times as required. As it is observed this only requires extensionof width of crystal without extending the length of the TGG crystalwhich can significantly reduce size and cost of the isolators orcirculators based on this configuration.

Referring now to FIGS. 7A and 7B, and in a preferred embodiment of thepresent invention, there is shown a semi-double isolator 225 having anisolation of over 70 dB. In practice, a slab of TGG crystal which issandwiched between two slab of magnets cannot produce large extinctionratio mainly due to residual stress at sharp corners of slab TGG crystaland non uniformity of field generated from slab magnets. Therefore, itis difficult to fabricate an isolator with large isolation, i.e., closeto 40 dB for single stage, using slab magnets and slab TGG crystalgeometry. To overcome this issue, semi-double isolator 225 is modifiedfrom the slab configuration to replace slab TGG crystal with cylindricalrod 230 and replace slab magnets with a piped-shaped magnet 235.Polarizers 240 and a reflector 245 are configured relative tocylindrical rod and piped-shaped magnet 225 for directing light 250therethrough.

1. An optical isolator for transmitting light in a first direction alongan optical pathway therethrough and blocking the light in a seconddirection along the optical pathway, and the first direction and thesecond direction being in opposition to one another, the opticalisolator comprising: an input polarizer and an output polarizer, theinput polarizer having a first pass axis of a first given angle, theoutput polarizer having a second pass axis of a second given angle, theinput polarizer configured to polarize the light entering into theoptical pathway to a first given plane of polarization parallel to thefirst given angle; a Faraday rotator material disposed between the inputpolarizer and the output polarizer, the Faraday rotator material havinga given Verdet constant, a first end and a second end in opposition toone another, the first end and the second end disposed at a maximumlinear distance across the Faraday rotator material from one another,and the first end and the second end defining an axis therebetweendefining a maximum linear length through the Faraday rotator material;generation means for generating a magnetic field around and inside theFaraday rotator material, the generation means providing a givenmagnetic field strength; and at least one reflector configured along theoptical pathway from the input polarizer to the output polarizer, the atleast one reflector defining a given optical length of the opticalpathway through the Faraday rotator material, and the given opticallength through the Faraday rotator material being longer than themaximum linear distance across the Faraday rotator material; wherein thegiven length of the optical pathway through the Faraday rotator materialprovided by the at least one reflector, the given magnetic fieldstrength provided by the generation means, and the Verdet constant ofthe Faraday rotator material are selected with respect to one another soas to rotate the light along the given length of the optical pathwaythrough the Faraday rotator material from the first given angle of theinput polarizer to the second given angle of the output polarizer.
 2. Anoptical isolator according to claim 1 wherein the difference between thefirst given angle and the second given angle is 45°.
 3. An opticalisolator according to claim 1 wherein the Faraday rotator materialcomprises magneto-optic crystal.
 4. An optical isolator according toclaim 3 wherein the magneto-optic crystal is Terbium Gallium Granite(TGG) crystal.
 5. An optical isolator according to claim 1 wherein thelight isolated by the Faraday rotator material has a wavelength of under1000 nm.
 6. An optical isolator according to claim 5 wherein thewavelength of the light is 976 nm.
 7. An optical isolator according toclaim 5 wherein the wavelength of the light is 980 nm.
 8. An opticalisolator according to claim 5 wherein the wavelength of the light is 880nm.
 9. An optical isolator according to claim 1 wherein the givenoptical length of the optical pathway through the Faraday rotatormaterial is at least twice the maximum linear distance across theFaraday rotator material.
 10. An optical isolator according to claim 1wherein the generation means comprise at least one magnet.
 11. Anoptical isolator according to claim 10 wherein the at least one magnetis round and is configured to surround at least a portion of the Faradayrotator material.
 12. An optical isolator according to claim 10 whereinthe at least one magnet is a pair of bar magnets disposed adjacent toand on opposing sides of the Faraday rotator material.
 13. An opticalisolator according to claim 10 wherein the at least one magnet is apermanent poled magnet.
 14. An optical isolator according to claim 10wherein the at least one magnet is an electromagnet.
 15. An opticalisolator according to claim 1 wherein the at least one reflectorcomprises a highly reflective coating disposed on the Faraday rotatormaterial.
 16. An optical isolator according to claim 15 wherein thehighly reflective coating is disposed on a first facet and a secondfacet of the Faraday rotator material so as to form a multipass etalon.17. An optical isolator according to claim 16 wherein the Faradayrotator material comprises an uncoated region on each of the first facetand the second facet, respectively, so as to allow light to enter andexit the multipass etalon.
 18. An optical isolator according to claim 1wherein the at least one reflector comprises a highly reflective mirror.19. An optical isolator according to claim 18 wherein the highlyreflective mirror is disposed adjacent to the Faraday rotator material.20. An optical isolator according to claim 19 wherein the highlyreflective mirror is disposed a given distance from the Faraday rotatormaterial.
 21. An optical isolator according to claim 1 furthercomprising selection means for selecting a given angle of incidence ofthe light disposed on the input polarizer, the selection meansconfigured to select the optical pathway through the Faraday rotatormaterial.
 22. An optical isolator according to claim 21 wherein theselection means provide an adjusted length of the optical pathway fromthe given length of the optical pathway.
 23. An optical isolatoraccording to claim 22 wherein the adjusted length of the optical pathwaycomprises a chosen number of reflections through the Faraday rotatormaterial.
 24. An optical isolator according to claim 23 wherein thechosen number of reflections through the optical pathway for theadjusted length are equal to a given number of reflections through theoptical pathway for the given length of the optical pathway.
 25. Anoptical isolator according to claim 23 wherein the chosen number ofreflections through the optical pathway for the adjusted length aregreater than a given number of reflections through the optical pathwayfor the given length of the optical pathway.
 26. An optical isolatoraccording to claim 1 wherein the light blocked in the second directionhas an isolation of greater than 50 dB.
 27. An optical isolatoraccording to claim 1 further comprising at least one additionalpolarizer disposed in the optical pathway between the input polarizerand the output polarizer.
 28. An optical isolator according to claim 27wherein the at least one additional polarizer comprises at least oneadditional reflector configured to redirect the optical pathway.
 29. Amethod of optically isolating light by allowing transmission of thelight in a first direction along an optical pathway through an opticalisolator and blocking transmission of the light in a second directionalong a second direction through the optical isolator, and the firstdirection and the second direction being in opposition to one another;the method comprising: initially polarizing the light with an inputpolarizer, the light being polarized at a first given plane ofpolarization parallel to a first given angle; transmitting the initiallypolarized light along an optical pathway through a Faraday rotatormaterial having a magnetic field applied thereto so as to rotate theinitially polarized light from the first given angle to an intermediateangle; reflecting the polarized light to provide a given number ofpasses through a portion of the Faraday rotator material so as tofurther rotate the polarized light from the intermediate angle to asecond given plane of polarization parallel to a second given angle; andpassing the polarized light at the second given plane of polarizationparallel to the second given angle through an output polarizer; whereinthe polarized light is reflected along the optical pathway between theinput polarizer and the output polarizer so as to provide an appropriatelength of the optical pathway with a reduced length of the Faradayrotator material.
 30. A method of optically isolating light, the methodcomprising: providing an optical isolator for transmitting light in afirst direction along an optical pathway therethrough and blocking thelight in a second direction along the optical pathway, and the firstdirection and the second direction being in opposition to one another,the optical isolator comprising: an input polarizer and an outputpolarizer, the input polarizer having a first pass axis of a first givenangle, the output polarizer having a second pass axis of a second givenangle, the input polarizer configured to polarize the light enteringinto the optical pathway to a first given plane of polarization parallelto the first given angle; a Faraday rotator material disposed betweenthe input polarizer and the output polarizer, the Faraday rotatormaterial having a given Verdet constant, a first end and a second end inopposition to one another, the first end and the second end disposed ata maximum linear distance across the Faraday rotator material from oneanother, and the first end and the second end defining an axistherebetween defining a maximum linear length through the Faradayrotator material; generation means for generating a magnetic fieldaround the Faraday rotator material, the generation means providing agiven magnetic field strength; and at least one reflector configuredalong the optical pathway from the input polarizer to the outputpolarizer, the at least one reflector defining a given optical length ofthe optical pathway through the Faraday rotator material, and the givenoptical length through the Faraday rotator material being longer thanthe maximum linear distance across the Faraday rotator material; whereinthe given length of the optical pathway through the Faraday rotatormaterial provided by the at least one reflector, the given magneticfield strength provided by the generation means, and the Verdet constantof the Faraday rotator material are selected with respect to one anotherso as to rotate the light along the given length of the optical pathwaythrough the Faraday rotator material from the first given angle of theinput polarizer to the second given angle of the output polarizer;polarizing the light entering the input polarizer to the first givenangle; rotating the polarized light from the first given angle to thesecond given angle through the Faraday rotator material; and passing thepolarized light from the Faraday rotator material through the outputpolarizer so as to prevent reflected light from transmission through theinput polarizer due to a non-reciprocal rotation of the light in thesecond direction through the Faraday rotator material so as to allow theinput polarizer to block the reflected light.